Referring to FIG. 1, a conventional BGA package substrate 100 is generally applied for carrying chip in well-known BGA packages and mainly comprises a substrate core 110, a plurality of ball pads 120 and a solder resist layer 130. The substrate core 110 has a surface 111, the ball pads 120 are disposed on the surface 111 of the substrate core 110, and the solder resist layer 130 is formed over the surface 111 of the substrate core 110 without covering the ball pads 120. Additionally, a plurality of traces 140 are disposed on the surface 111 of the substrate core 110 to connect the ball pads 120 and encapsulated by the solder resist layer 130.
In addition to the substrate 100, the known BGA package further comprises a plurality of solder balls 210, a chip 220 and an encapsulant 230. The solder balls 210 are mounted onto the ball pads 120 of the BGA package substrate 100. The chip 220 is disposed on the BGA package substrate 100 and has a plurality of bonding pads 221. The bonding pads 221 are further connected to the fingers of the traces 140 on the BGA package substrate 100 for connecting a plurality of bonding wires 240 formed by wire-bonding method. The encapsulant 230 encapsulates the chip 220 and the bonding wires 240. The solder balls 210 are applied for mounting the BGA package to an external printed circuit board 250. Referring to FIG. 2, the circuit board 250 has a plurality of ball-mounting pads 251 and a solder resist layer 252 formed thereon. The solder balls 210 are reflowed to bond to the corresponding ball-mounting pads 251.
However in the known BGA package, disposition of the ball pads 120 will become higher and higher in density, especially for the ball pads 120 of NSMD pad (Non-Solder Mask Defined pad). The periphery of the ball pads 120 are not covered by the solder resist layer 130. The smaller the area of that each of the ball pads 120 occupies the surface 111 of the substrate core 110 is, more inadequate immobilization the ball pads 120 have on the substrate core 110. When a BGA package is connected with SMT to a printed circuit board or a board level temperature cycle test, it is possible to easily cause a separation or crack between the ball pads 120 and the substrate core 110 resulting in serious problems such as bad reliability or component failure.
A BGA package substrate has been disclosed in U.S. Pat. No. 6,201,305 B1, which integrally adds some bar supporters in radial shape extending to inside of the solder resist layer 252 so as to improve immobilization of ball pads. However, the area of that the ball pads occupy the substrate surface will be widely increased by doing in this way, so that it is unsuitable for BGA package substrate with high density disposition and may further affect wiring disposition of BGA package substrate.